1. Field of the Invention
The invention relates generally to digital PWM amplifiers and more particularly to systems and methods for maintaining minimum pulse width during shutdown.
2. Related Art
Audio amplifiers are designed to receive input signals that typically have very low voltages and/or currents, and to generate corresponding output signals that generally have much higher voltages and/or currents. Because of these high voltages and/or currents, conditions may develop which present a danger to the amplifier and/or speakers.
For example, in a pulse-width modulation (PWM) amplifier, preventing the amplifier from generating excessive output current (creating an over-current condition) is one of the most critical functions, since this could damage the amplifier's output stages or the speakers driven by the output stages. The amplifier can avoid over-current conditions is various ways. Probably the most straightforward solution is to simply shut down the system whenever an over-current condition occurs.
The fastest and most dramatic way to shut down a PWM output stage is to tristate the PWM modulator (deassert the PWM signals that drive output FETs). The act of tristating the modulator, however, may violate a minimum pulse width requirement for the PWM signals output by the modulator. If the minimum pulse width requirement is violated, certain output stage designs, such as those using capacitively coupled output stages, may have error conditions that destroy the output stage.
Conventional solutions rely on an ad-hoc series of logic blocks after the shutdown logic to prevent MPW (Minimum Pulse Width) violations. These MPW compensation logic blocks normally examine pulses produced by the PWM modulator to determine whether they have widths which are less than a minimum threshold. If a pulse width is less than the minimum threshold, the MPW compensation blocks typically stretch the pulses so that they no longer violate the MPW requirement. This stretching of the pulses, however, may have other adverse consequences, such as cross-conduction in the stretched region (since both PWMH and PWML are asserted). This could be corrected with additional compensation circuitry, but the correction for cross-conduction may also cause subsequent minimum pulse width violations. It would therefore be desirable to provide systems and methods for preventing minimum pulse width violations which do not suffer from these drawbacks.